The present invention relates to particulate filters for the removal of particulates from combustion engine exhaust gases, and methods for using such filters, that provide substantial improvements in filter durability and efficiency.
Wall flow diesel particulate filter technology has been used with some success for approximately 15 years. The filters provide excellent filtration of particulate matter, but have experienced problems during the regeneration phase of their operation. The regeneration phase is required to burn the complement of accumulated soot out of the filter. This process is typically accomplished by when the internal temperature of an uncatalyzed filter reaches approximately 600° C. When the carbon soot particles reach this temperature in the presence of adequate oxygen the combustion process can proceed.
The rate and severity of the exothermic temperatures reached during the filter regeneration reaction can be a problem. The problem arises in part because the amounts and types of soot accumulated prior to initiating soot regeneration cannot be accurately predicted. At present, engine manufacturers utilize various criteria to indicate the need for a regeneration cycle, one approach being to simply correlate soot accumulation with a one-dimensional variable such as filter pressure drop observed on an engine test stand. This approach does not take into account the fact that the variety of engine operating conditions that can exist make both the nature of the soot accumulation and the combustion behavior of that accumulated soot highly variable.
In cases where excessive concentrations of carbon soot particles are accumulated in the filter the maximum exotherm temperatures can be high enough to melt cordierite. i.e., reaching reaction temperatures of 1450° C. or more. This melting leads to failure of the filter due to particulate leakage. In addition, the extreme nature of some regenerations may induce filter cracking due to the large thermal gradient within the part, also resulting in particulate leakage. And finally, it may be desirable to limit the maximum filter regeneration temperature, e.g., to temperatures of 1000° C. or lower, for purposes such as protecting the catalyst against deactivation. For all of these reasons it is critical to control the regeneration reaction to achieve maximum filter durability.
Without knowing the conditions that will damage the filters, engine and engine control designers cannot keep the engine output from creating uncontrolled exothermic reactions during regeneration. This inability to understand and control the regeneration process to better manage filter performance prevents less costly filter materials from be used to meet the filtration needs of the automotive industry. Instead expensive materials solutions have to be considered that lead to undesirable tradeoffs in fuel efficiency and emissions
The current technique being adopted for mitigating these extreme regenerations is to choose a filter material that will not yield extreme regenerations in the presence of high soot concentrations. The reason is that it may not always be possible to limit the amount of accumulated soot. For this reason SiC filters have been tapped for early introduction in the marketplace.
SiC filters can demonstrate improved thermal capacity under conditions of high soot accumulation for two primarily reasons. The first reason is that SiC wall flow filters produced at an equivalent wall thickness and cell density to cordierite will respond with lower exotherm reactions for an equal complement of accumulated soot. Silicon carbide has a phase density that is greater than that of cordierite and therefore possesses greater capacity to absorb heat without experiencing a temperature rise as compared to an equivalent cordierite filter. Another reason for the lower exothermic response is that under identical inlet gas temperatures the SiC filter will start its exothermic reaction at a lower temperature than that for cordierite. The higher thermal conductivity of SiC compared to cordierite permits it to loose heat to the surrounding environment more efficiently and therefore operates at a lower equilibrium filter temperature for a given set of inlet temperatures.
Another concern, applicable to all prospective exhaust filters, is that the regeneration process once initiated be supported sufficiently to proceed to completion during each cycle, in order to reduce need for more frequent regeneration cycling. Current regenerating schemes can permit incomplete regeneration due to insufficient combustion temperatures, since the engine output conditions required to initiate full regeneration can differ based on both soot combustion and filter performance characteristics.
The underlying problem is that present filter regeneration cycles are initiated with limited information about actual filter response, precluding overall design optimization for reduced regeneration cycle frequency and overall reduced system cost. Much effort has been directed at adapting filter composition and geometry as above described, but little work has been done to understand and control filter response over the wide variations of exhaust input conditions these filters may experience.
The recent paper, “Control of Oxygen for Thermal Management of Diesel Particulate Filters”, T. Brewbaker et al., SAE Technical Paper Series 2002-01-0427, SAE 2002 World Congress, Detroit, Mich. (March 2002), suggests a method for avoiding excessive filter regeneration temperatures by limiting the oxygen supplied to the filter during regeneration. This is accomplished by adapting the engine exhaust gas recirculation (EGR) system, a system primarily designed to limit engine combustion temperatures, so that additional exhaust gas is recirculated into the combustion mixture during the filter regeneration cycle. While this adaptation has some effect on limiting filter regeneration temperatures, it does not take into account the actual response of the filter to other changes in exhaust gas characteristics. Thus factors such as reduced exhaust temperature or increased exhaust soot content that can significantly affect the severity and/or efficiency of the regeneration cycle are not considered.